1. Field of the Invention
This invention relates to oxygen enrichment in industrial heating applications, in particular in power boilers.
2. Related Art
In a previous disclosure of the same assignee of the present invention, Air Liquide Serie file 5167, filed Nov. 10, 1999, application Ser. No. 09/437,526, now U.S. Pat. 6,314,896 issued Nov. 13, 2002, there was a proposed scheme of oxygen-enrichment in boilers using large amounts of oxygen, up to full oxy-fuel combustion. That patent application involved a certain ratio between the oxygen enrichment and the flue gas recirculation, such that the design boiler parameters are maintained constant.
While quite inventive, the above-referenced methods in said patent application may not be desirous in particular industrial heating applications, in particular coal fired boilers, primarily pulverized coal, but with the application to fluidized beds also. Coal combustion results in a potentially large amount of unburned coal in the stack, thus losing a large amount of fuel. Also, due to the incomplete combustion of coal, as well as to the sometimes difficult ignition process, a support fuel such as natural gas is frequently used in significant quantities (from about 10% to about 50% of the total amount of fuel). The ease and completeness of combustion is directly related to the volatile content of the coal, and indirectly related to the percentage of moisture in the coal. In other words, more moisture means more difficult and possibly incomplete combustion, while more volatiles in the coal means more complete combustion.
FIG. 1 presents in schematic, a prior art combustion chamber 2 in a boiler 4, where the combustion chamber 2 is divided into two major zones: Zone I, as denoted in FIG. 1 at 6, represents the area where combustion burners are located, together with air inlets. Combustion air can enter combustion chamber 2 together with a fuel (part of the air is used to transport coal into combustion chamber 2), or in different inlets. Combustion air can be introduced into the boiler partially or totally at this location. More modern schemes use a different air inlet, denoted as Zone II, and noted as 8 in FIG. 1, in order to improve the combustion process and to lower the NOx emissions. The combustion scheme illustrated schematically in FIG. 1 is termed “staged combustion” since the combustion process occurs in two zones. It is noted that the schematic in FIG. I is very general, showing a generally horizontal flue gas circulation 10. In general, flue gas circulation can be in any direction (vertical, horizontal, circular, and the like).
Another aspect of the invention is a method of increasing combustion of a first hydrocarbon fuel in a combustion chamber, the combustion normally using only air as an oxidant, part of the air entering the combustion chamber through one or more first hydrocarbon fuel burners in a first zone of the combustion chamber, and a remaining portion of air normally entering the combustion chamber at a plurality of locations downstream of said first hydrocarbon fuel burners, the method comprising injecting a first oxygen-enriched gas into the combustion chamber at said plurality of locations, the first oxygen-enriched gas being in an amount sufficient to provide an oxygen concentration of no more than 2% greater than when said air is used alone, and wherein the first oxygen-enriched gas is injected through a lance at a velocity, said lance injecting said first oxygen-enriched gas into a flame created by a second oxygen-enriched gas and a second hydrocarbon fuel.
As illustrated in FIG. 1, the combustion process is divided into two major zones: Zone I represents the ignition zone, where the fuel(s) enter the combustion chamber, are heated, and ignite. When coal is of a lesser quality, additional fuel (generally natural gas or fuel oil) is required for a fast ignition. Zone II represents the final region allocated for combustion. Additional oxidant may be introduced, as mentioned supra. Modern staged combustion schemes allow a significant portion of the oxidant to enter Zone II (between 10% and 50% of the total oxidant). Due to the low pressure of the incoming air in Zone II, the flow patterns of the main flue gas stream are relatively undisturbed, thus the mixing between the two streams is relatively poor, preventing the full combustion of the fuel. This is represented by the shaded area 16 in FIG. 2. FIG. 2 illustrates that the mixing zone 16 between the flue gas stream 10 and the balance of the oxidizer in Zone II and exiting into the mixing zone 16 is not total, thus an important part of the fuel will not mix with the oxidant, thus remaining unburned.
It would be an advantage if the fuel from Zone I could be mixed with oxidant from Zone II to provide better mixing between fuel and secondary oxidant.